Urea ammonium nitrate production

ABSTRACT

The invention relates to a process for the production of urea ammonium nitrate, a system and a method of modifying a plant. The process comprises treating ammonia-containing off-gas resulting from the production of ammonium nitrate (AN off-gas) with acidic scrubbing liquid in a finishing treatment section having a gas inlet in fluid communication with a gas outlet of a finishing section of a urea production unit, wherein the finishing section is adapted to solidify urea liquid, and wherein said finishing treatment section is adapted to subject ammonia-containing off-gas of the finishing section to treatment with an acidic scrubbing liquid.

The invention is in the field of the production of a solution of ureaammonium nitrate in water (UAN). The invention also pertains to a systemfor producing UAN and to a method of modifying a plant.

BACKGROUND OF THE INVENTION

Urea ammonium nitrate (UAN) is an aqueous solution of urea and ammoniumnitrate and is used as fertilizer. A process for producing UAN generallycomprises producing ammonium nitrate in an ammonium nitrate section (ANproduction section), forming urea in a urea production unit, andcombining said produced ammonium nitrate and urea to produce ureaammonium nitrate in a urea ammonium nitrate section (UAN productionsection).

The production of ammonium nitrate solution involves the neutralisationreaction of gaseous ammonia with concentrated nitric acid solutionaccording to the following reaction:

HNO₃+NH₃→NH₄NO₃

The production of ammonium nitrate solution generally results in anoff-gas (AN off-gas), in particular from the neutralization reaction ofammonia with nitric acid. Because the formation of ammonium nitrate is ahighly exothermic reaction, the AN off-gas generally comprises watervapor, residual ammonia and entrained droplets from the reaction medium.The droplets can comprise ammonium nitrate and/or nitric acid.Accordingly, AN off-gas may for instance comprise CO₂, NH₃, water,ammonium nitrate, N₂, O₂ and nitric acid. The AN off-gas is usuallysubjected to condensation and the condensate is for instance passed inpart to the UAN production section.

Some general desires for improving a process comprising producing UANinclude reducing steam consumption (increasing energy efficiency) andreducing emissions and emission points, in particular of ammonia. It isalso desired to reduce waste streams, improve product recovery andreduce the amount of make-up water and acid used for scrubbing. Suchadvantages are desirably obtained for new plants (grass-roots plants) aswell as by modifying or revamping existing plants. Equipment costs andother capital expenditures are preferably minimized.

SUMMARY OF THE INVENTION

In order to better address one or more of the foregoing desires, theinvention, in one aspect, provides a process for the production of ureaammonium nitrate, comprising:

(a) subjecting ammonia and carbon dioxide to urea forming conditions soas to obtain an aqueous urea solution,

(b) purifying the aqueous urea solution in a recovery section to removeresidual ammonium carbamate so as to form a purified aqueous ureasolution, and optionally subjecting at least part of the purifiedaqueous urea solution to evaporation so as to form concentrated urealiquid (urea melt),

(c) subjecting ammonia and nitric acid to ammonium nitrate formingconditions so as to form an aqueous ammonium nitrate solution;

(d) combining said aqueous ammonium nitrate solution and at least a partof the purified aqueous urea solution and/or concentrated urea liquid ina urea ammonium nitrate section so as to obtain an aqueous solution ofurea ammonium nitrate;

(e) treating ammonia-containing off-gas resulting from the production ofammonium nitrate (AN off-gas) with acidic scrubbing liquid in afinishing treatment section having a gas inlet in fluid communicationwith a gas outlet of a finishing section of a urea production unit,wherein the finishing section is adapted to solidify urea liquid,wherein said finishing treatment section is adapted to subjectammonia-containing off-gas of the finishing section to treatment with anacidic scrubbing liquid.

The invention also relates to a system for the production of at leasturea and urea ammonium nitrate, comprising:

-   (a) a urea production unit comprising a finishing section, wherein    the finishing section has a gas outlet for ammonia-containing    off-gas,-   (b) an ammonium nitrate section for producing ammonium nitrate in    fluid communication with a source of nitric acid and a source of    ammonia, having an outlet for aqueous ammonium nitrate solution and    an outlet for off-gas,-   (c) a urea ammonium nitrate section comprising a unit having an    inlet in fluid connection with said outlet for aqueous ammonium    nitrate solution and an inlet for receiving urea liquid, for    combining said ammonium nitrate solution and said urea liquid, and    having an outlet for urea ammonium nitrate solution, and-   (d) a finishing treatment section having a gas inlet in fluid    connection with said gas outlet for off-gas of said finishing    section, adapted to subject ammonia-containing off-gas of the    finishing section to treatment with an acidic scrubbing liquid,-   wherein said finishing treatment section comprises a gas inlet in    fluid communication with said outlet for off-gas of said ammonium    nitrate section.

The invention also relates to a method of modifying a plant, wherein theplant comprises an ammonium nitrate section for reacting ammonia andnitric acid under ammonium nitrate forming conditions, a finishingsection adapted to solidify a urea liquid, and a finishing treatmentsection having a gas inlet in fluid communication with a gas outlet ofsaid finishing section, adapted to subject ammonia-containing off-gas ofthe finishing section to treatment with an acidic scrubbing liquid,wherein the method comprises adding a connection for fluid communicationbetween an outlet for off-gas from said ammonium nitrate section and angas inlet of said finishing treatment section, such as piping or tubing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a process scheme for a non-limiting example of a processand system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is based on the judicious insight to integrate thetreatment of ammonia-containing off-gas resulting from the production ofammonium nitrate (AN off-gas) with a treatment of ammonia-containingoff-gas of a finishing section (finishing off-gas) with an acidicscrubbing liquid.

The process of the invention accordingly comprises treatingammonia-containing off-gas resulting from the production of ammoniumnitrate (AN off-gas) with acidic scrubbing liquid in a finishingtreatment section.

In the process, AN off-gas is sent to a gas inlet of the finishingtreatment section so as to be subjected to treatment with the acidicscrubbing liquid. Preferably, at least part of the AN-off gas issupplied into the finishing treatment section. Preferably at least partof the AN off-gas is contacted with, combined with and/or mixed withfinishing off-gas that is supplied to the finishing treatment section.Hence, at least part of the AN off-gas is treated in a finishingtreatment section. In this way, the integration may provide for moreefficient treatment of AN off-gas and/or finishing off-gas.

In contrast to prior art processes, a step of condensing the AN off-gascan be omitted. This allows for eliminating the condenser used in someprior art processes.

The AN off-gas comprises at least ammonia when supplied into thefinishing treatment section. In the process, ammonia-containing ANoff-gas is sent to a gas inlet of said finishing treatment section so asto be subjected to treatment with the acidic scrubbing liquid in thefinishing treatment section.

In the process, at least part of the AN off-gas enters the finishingtreatment section as gaseous stream (which may contain droplets andentrainments). The process can for example comprise combining a gaseousAN off-gas stream with gaseous ammonia-containing off-gas of thefinishing section. The gaseous AN off-gas stream can for example havethe same composition at the gas inlet of the finishing treatment sectionas at the outlet of the AN production section. Preferably, at least 50vol % of the AN off-gas, more preferably at least 90 vol %, oressentially all of the AN off-gas stream as obtained from the ANproduction section is combined as gaseous stream with a finishingoff-gas stream to give a combined gaseous stream. The combined gaseousstream is treated in the finishing treatment section. The AN off-gasstream may amount to for example from 0.5 to 10 vol % of the finishingoff-gas, such as from 1.0 to 5.0 vol. %, although the process is notlimited to a particular ratio between AN off-gas and finishing off-gas.The relative amount of AN off-gas depends for instance on the capacityof the urea finishing section.

Advantageously, treatment of AN off-gas in a finishing treatment sectionallows for a net reduction of ammonia emission. The process of theinvention surprisingly allows for treating additional AN off-gas in thefinishing treatment section with no or only a small increase of theemission of ammonia and urea dust from the finishing treatment section.Advantageously, aerosols in the AN off-gas can be captured in thefinishing treatment section, for example by a scrubber such as a venturiscrubber, which can for example be additional to the scrubber used forscrubbing with acid liquid. Since the finishing treatment section isgenerally designed for removing particles from the finishing off-gas, itcan advantageously be used for removing aerosol droplets from the ANoff-gas, furthermore in combination with ammonia removal from the AN-offgas.

In addition, the presence of urea dust in the off-gas from the finishingsection may provide more feeding material to enlarge particles entrainedin AN off-gas during a quench step, if used, thereby further improvingthe removal of such particles.

Hence, the process of the invention allows for eliminating the ammoniaemission from the conventional separate treatment section for ANoff-gas, which can for example be about 25 mg ammonia/Nm³ dry air. Inthis way, ammonia emission from the AN production section can bedecreased or eliminated while ammonia emission from the finishingtreatment section can for example remain the same, or possibly decrease,or increase less than the ammonia emission decrease for the ANproduction section. In this way, the process may allow for a reductionof total ammonia emission and emission points.

Moreover, with the process of the invention advantageously lessequipment is required for the AN production section, for example byeliminating a scrubber and condenser for AN off-gas in the AN productionsection, and auxiliary equipment thereof such as pumps. Hence, in apreferred embodiment, all or essentially all of the AN off-gas is passedto the finishing treatment section as gaseous stream without anintermediate step of condensing, or without an intermediate step ofscrubbing, or even without any intermediate steps yielding liquidstreams. By eliminating a separate scrubber for the AN off-gas, the needfor recovering ammonium nitrate and nitric acid from scrubbing liquidthereof is also eliminated. By eliminating the condenser of the ANproduction section no condensate needs to be treated.

In a preferred embodiment, the process does not involve condensingoff-gas resulting from the production of ammonium nitrate between theurea ammonium nitrate section and the finishing treatment section.

The process also allows for reduced water consumption by a moreefficient use of make-up water, as a combined stream is treated, forexample by 10-20%. The process further allows for a reduction of powerconsumption, for example of up to 2 kWh/ton, such as compared to aprocess with a separate scrubber for AN off-gas.

The finishing section is for example the finishing section of the ureaproduction unit wherein aqueous urea solution and/or concentrated urealiquid are produced. In another embodiment, the finishing section is afinishing section of a second urea production unit from which no urealiquid is supplied to the urea ammonium nitrate section. In such case,the first urea production unit does not necessarily have a finishingsection.

The aqueous urea solution, the purified aqueous urea solution and theconcentrated urea liquid are some of the streams of urea liquid in theprocess of the invention. The term “urea liquid” can also refer to aurea-containing liquid stream of a second urea production unit.

Preferably, the process comprises recycling scrubbing liquid utilized inthe finishing treatment section to the UAN production section.Preferably, the scrubbing liquid is recycled such that it is at least inpart included in the aqueous solution of urea ammonium nitrate. Theprocess may for example comprise combining scrubbing liquid utilized inthe finishing treatment section with the aqueous ammonium nitratesolution, the purified aqueous urea solution and/or concentrated urealiquid.

The process comprises subjecting ammonia-containing off gas of thefinishing section and AN off-gas to a treatment with an acidic scrubbingliquid, in particular in the same finishing treatment section.Accordingly, various components of both the off-gas streams are removedfrom the off-gas and end up in a liquid stream (liquid recycle stream),in particular urea and ammonium nitrate. For example, the scrubbing usesa circulating solution as scrubbing liquid, with additional make-upwater. A purge flow is obtained from the scrubber, usually with 10%-60%urea by weight, which provides the liquid recycle stream.

The process preferably comprises including at least part of the liquidrecycle stream, such as all, in the UAN solution. Preferably, at leastpart of the liquid recycle stream, such as all, is combined with anammonium nitrate containing stream in the UAN production section, forexample in the mixing unit. Herein, the liquid recycle stream refers tothe scrubbing liquid utilized in the finishing treatment section afterwithdrawal from that section.

This preferred embodiment advantageously avoids the need for a separatetreatment of said liquid recycle stream. Moreover, this embodimentallows for recovering for instance ammonium nitrate and/or urea from theoff-gasses, thereby allowing for a higher UAN production. Thisembodiment may also allow for recovering nitric acid from the ANoff-gas, thereby allowing for reduced nitric acid consumption in the ANproduction section. In addition, it can allow for adjusting theconcentration of the UAN solution. A particular advantage of thisembodiment is that urea in the finishing off-gas can optionally berecovered into UAN rather than in a urea product stream. Accordingly,the process allows for eliminating the recycling of any additives addedto the urea, such as anti-caking agents and granulation aids, inparticular formaldehydes, back into a urea production unit.

Recycling scrubbing liquid utilized in the finishing treatment sectionto the UAN production section is in particular advantageous if a part ofthe urea liquid is used as very pure urea product or is used for theproduction of such urea product. Examples of such urea products areDiesel Exhaust Fluid (DEF) and urea products which are suitable for thepreparation of DEF by adding demineralized water (together referred toas DEF products). DEF is generally an aqueous urea solution with maximum0.3 wt. % biuret and maximum 0.2 wt. % of alkalinity, in particular <0.2wt. % ammonia, with 32.5 wt. % urea. DEF is injected in the tail gas ofcombustion engines to react with NO_(x) to reduce NO_(x) emission.Presence of contaminations in DEF is not desirable; in particular thepresence of formaldehyde and other anti-caking agents is unwanted. DEFis generally produced by diluting a urea liquid, such as purifiedaqueous urea solution, or by dissolving urea granules with demineralizedwater. The urea liquid or urea granules accordingly should also have avery low level of biuret, alkalinity and other contaminations.

Accordingly, the process preferably comprises a step of preparing a ureaproduct from a part of a stream of urea liquid of the urea productionunit having the finishing section, wherein the urea product is a DEFproduct. Preferably, the DEF product has maximum 0.3 wt. % biuret andmaximum 0.2 wt. % of alkalinity, in particular <0.2 wt. % ammonia, andcomprises maximum 0.10 wt. % additives, preferably less than 0.010 wt. %additives, or for example less than 0.010 wt. % formaldehyde, and atleast 30 wt. % urea, based on total weight of the product. Preferably,the product is essentially free of formaldehyde. Preferably the processfurther comprises adding an additive to another part of said stream ofurea liquid, for example an anti-caking agent such as formaldehyde.Hence, a preferred process comprises dividing a stream of urea liquid ofthe urea production unit having the finishing section in at least twoparts, and adding an additive to one of said streams downstream of saiddividing, wherein downstream is defined with respect to said stream, andpreparing a DEF product from the other stream, wherein said additive ispreferably formaldehyde, and wherein the stream with said additive issubjected to solidification in the finishing section, wherein thesolidification is preferably granulation. In such process, the off-gasfrom the solidification is scrubbed in the finishing treatment sectionand the liquid recycle stream obtained with the scrubbing is included inthe UAN product, for example supplied to the UAN production section.

Accordingly, a process for the production of a solid urea product, a DEFproduct and UAN preferably comprises producing UAN and a urea liquid,using a part of the urea liquid for producing a DEF product, and usinganother part of the urea liquid for producing a solid urea product byadding an additive, such as formaldehyde, and solidifying the urea, suchas by granulating, wherein finishing off-gas is obtained. The off-gascomprises urea, ammonia and said additive. At least part of the urea isrecovered from the off-gas into a recycle stream, for example byscrubbing, and at least part of said recycle stream is included in theUAN product. The process preferably does not comprise supplying therecycle stream into a stream from which the DEF product is prepared.This process can in principle be used independently of the feature ofsupplying AN off-gas to a finishing treatment section, although thecombination is preferred.

A suitable process for DEF preparation is described in for exampleEP1856038A1 and comprises using a urea aqueous solution obtaineddirectly from or after the recovery section of the urea melt plant, anddiluting the urea aqueous solution with water to obtain the desiredsolution. In terms of the present invention, for example the purifiedaqueous urea solution can be diluted to prepare DEF.

The process comprises subjecting ammonia and carbon dioxide to ureaforming conditions so as to obtain an aqueous urea solution.

The urea can be synthesized by any suitable method. A frequently usedprocess for the preparation of urea according to a stripping process isthe carbon dioxide stripping process as for example described inUllmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp333-350. In this process, a high pressure synthesis section is followedby one or more recovery sections. The synthesis section comprises forexample a reactor, a stripper, and a condenser. The synthesis section isoperated at high pressure, such as between 12 and 18 MPa and preferablyin between 13 and 16 MPa. In the synthesis section the urea solutionleaving the urea reactor is fed to a stripper in which a large amount ofnon-converted ammonia and carbon dioxide is separated from the aqueousurea solution. Such a stripper can be a shell and tube heat exchanger inwhich the urea solution is fed to the top part at the tube side and acarbon dioxide feed to the synthesis is added to the bottom part of thestripper. At the shell side, steam is added to heat the solution. Theurea solution leaves the heat exchanger at the bottom part, while thevapor phase leaves the stripper at the top part. The vapor leaving saidstripper contains ammonia, carbon dioxide and a small amount of water.Said vapor is condensed by formation of ammonium carbamate in a fallingfilm type heat exchanger or a submerged type of condenser that can be ahorizontal type or a vertical type. A horizontal type submerged heatexchanger is described in Ullmann's Encyclopedia of IndustrialChemistry, Vol. A27, 1996, pp 333-350. The heat released by theexothermic carbamate condensation reaction in said condenser is usuallyused to produce steam that is used in a downstream urea processingsection for heating and concentrating the urea solution. Since a certainliquid residence time is created in a submerged type condenser, a partof the urea reaction takes already place in said condenser. The formedsolution, containing ammonium carbamate, water and urea together withthe non-condensed ammonia, carbon dioxide and inert vapor is sent to thereactor. In the reactor the above mentioned reaction from carbamate tourea approaches the equilibrium. The ammonia to carbon dioxide molarratio in the urea solution leaving the reactor is generally in between2.5 and 4 mol/mol. It is also possible that the condenser and thereactor are combined in one piece of equipment (for example a poolreactor). An example of this piece of equipment is described inUllmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp333-350. The formed urea solution leaving the urea reactor is suppliedto the stripper and the inert vapor containing non-condensed ammonia andcarbon dioxide is for example sent to a scrubbing section operating at asimilar pressure as the reactor. In that scrubbing section the ammoniaand carbon dioxide is scrubbed from the inert vapor. The formedcarbamate solution from the downstream recovery system is used asabsorbent in that scrubbing section.

Said vapor from said reactor can for example also be sent directly tothe ammonium nitrate section for neutralization (i.e. reaction withnitric acid so as to form ammonium nitrate). The urea solution leavingthe stripper in this synthesis section can for example have a ureaconcentration of at least 45% by weight and preferably at least 50% byweight thereby allowing for treatment in a single recovery systemdownstream the stripper. This urea solution is referred to as aqueousurea solution. These preferences also apply for the second ureaproduction unit, if used.

In the process of the invention, at least part of the aqueous ureasolution is purified in a recovery section. In this section, aqueousurea solution is purified to remove residual ammonium carbamate so as toform a purified aqueous urea solution. The recovery section comprisesfor example a heater, a liquid/gas separator and a condenser. Thepressure in this recovery section is for instance between 200 to 6000kPa. For example, a low pressure recovery section (2-7 bar) can be used,or a medium pressure recovery section (12-40 bar) followed by a lowpressure recovery section. In the heater of the recovery section thebulk of ammonia and carbon dioxide is separated from the urea and waterphase by heating the urea solution. Usually steam is used as heatingagent. The purified aqueous urea solution contains a small amount ofdissolved ammonia and carbon dioxide and leaves the recovery section.Optionally, at least part of the purified aqueous urea solution is sentto a downstream urea processing section, also referred to as ureaevaporation section or evaporation section. Herein, purified aqueousurea solution is optionally subjected to evaporation of water so as toform a concentrated urea liquid that is generally referred to as a ureamelt.

The invention is not limited to any particular urea production process.Other processes and plants include those that are based on technologysuch as total recycle plants, the HEC process developed by Urea Casale,the ACES process developed by Toyo Engineering Corporation and theprocess developed by Snamprogetti. All of these processes, and others,may be used in the process of the invention.

The process of the invention comprises subjecting ammonia and nitricacid to ammonium nitrate forming conditions so as to form an aqueousammonium nitrate solution. This step also results in an off-gas. Theunit, reactor or section in which this reaction is carried out, can bereferred to as AN production section.

Ammonium nitrate can for example be produced by reacting ammonia with astrong solution of nitric acid while maintaining the pH of the solutionwithin narrow boundaries Ammonia is for example used in its anhydrousform as gas and the nitric acid is for example concentrated (typicalconcentration range: 40 to 80 wt. %, such as about 60 wt. %). Via anexothermic neutralization reaction, the ammonium nitrate solution isreadily formed, typically at a concentration of about 70% to 95%, suchas 83% to 88%, for example with pH of about 7.

The AN production section can for instance comprise a neutralizerreactor based on a U-type combination of a circulator tube and mixingtube with a separation vessel reactor. The reaction is for exampleperformed at 0.15 MPa and 135-165° C.

Optionally, the process may comprise preparing solid ammonium nitrateproducts from a part of the ammonium nitrate, such as prills orgranules, wherein preferably the excess water is evaporated to anammonium nitrate (AN) content of 95% to 99.9% concentration.

Nitric acid used in the production of ammonium nitrate can be obtainedas an external feed. Preferably, the nitric acid is produced on site.Accordingly, the process optionally comprises oxidizing anhydrousammonia to nitric oxide, for example in the presence of a catalyst, andreacting nitric oxide with oxygen to form nitrogen dioxide. The processoptionally comprises absorbing nitrogen dioxide in water to form nitricacid and nitric oxide, or reacting nitrogen dioxide with oxygen andwater to form nitric acid. Accordingly, the system for the process ofthe invention optionally comprises a unit for the production of nitricacid. This unit will generally be fed from external sources and has anoutlet for nitric acid that is in fluid communication with an inlet fornitric acid of the unit for the production of ammonium nitrate.

Ammonia used in the AN production section can for example at least inpart be obtained from off-gas of the urea production unit and/or fromthe UAN production section. Preferably, also ammonia-containing off-gasfrom the urea production unit and/or from the urea ammonium nitratesection is sent to a gas inlet of the finishing treatment section.

Accordingly, the process may comprise supplying to the AN productionsection ammonia containing off-gas, such as overhead vapors, from theurea synthesis section, the recovery section, and/or the ureaevaporation section.

The off-gas may for instance be obtained from a recovery section of theurea production unit, wherein ammonium carbamate in the urea synthesissolution is decomposed to carbon dioxide and ammonia, typically at lowpressure (0.1-1 MPa, in particular 0.2-0.7 MPa), and/or at mediumpressure (1-4 MPa, preferably 1.5-3.0 MPa).

The process optionally further comprises passing off-gas from a sectionof the urea production unit other than the finishing section, forexample from the urea evaporation section, to the finishing treatmentsection. For example, at least part of a stream comprising water vaporevaporated in the urea evaporation section can be supplied to thefinishing treatment section.

The process comprises combining aqueous ammonium nitrate solution and atleast some urea liquid in a urea ammonium nitrate section (UANproduction section) so as to obtain an aqueous solution of urea ammoniumnitrate (UAN).

In the process of the invention, a part of the purified aqueous ureasolution and/or concentrated urea liquid is combined with aqueousammonium nitrate solution. Accordingly, the aqueous ammonium nitratesolution is combined with purified aqueous urea solution in case theprocess does not involve subjecting at least part of the purifiedaqueous urea solution to evaporation, and the aqueous ammonium nitratesolution is combined with purified aqueous urea solution and/orconcentrated urea liquid if the process involves subjecting at leastpart of the purified aqueous urea solution to evaporation. However, thisis not essential and that in principle urea supplied to the UANproduction section can be obtained from any source.

In a preferred embodiment a stream of concentrated urea liquid suppliedto the UAN production section comprises a minor part of the total amountof urea liquid supplied to the UAN production section. This allows foradjustment and control of the urea concentration in the UAN productionsection.

Optionally, a part of the purified aqueous urea solution obtained fromthe recovery section of the urea production unit can be supplied to theUAN production section, whereas another part can be supplied to theevaporation section and subsequently to the finishing section.Accordingly, the urea concentration of the urea liquid supplied to theUAN production section can for example be different from the ureaconcentration of urea liquid supplied to the finishing section.

The UAN production section preferably comprises a mixing unit, forinstance comprises a static mixer or a pipe mixer, for example a seriesof static mixers. The process preferably comprises mixing aqueousammonium nitrate solution and urea liquid.

The UAN products obtained with the present process contain for example28 wt. % to 32 wt. % of total nitrogen and typically of from 29 wt. % to38 wt. % urea and of from 36 wt. % to 48 wt. % of ammonium nitrate, withthe remainder being water.

In the process of the invention, a urea production unit comprises afinishing section adapted to solidify urea liquid. Usually, the urealiquid is concentrated before being subjected to solidification. Thefinishing section is for example the finishing section of the ureaproduction unit wherein aqueous urea solution and/or concentrated urealiquid are produced.

The process optionally comprises solidifying urea liquid in thefinishing section.

The solidification in the finishing section results inammonia-containing off gas (finishing off-gas). The solidification forexample comprises prilling, granulation, and/or pastillation of urea inthe finishing section. The solidification in the finishing sectionpreferably comprises exposing concentrated urea liquid to an air streamso as to obtain solid urea particles. The use of such air stream resultsin finishing off-gas. Accordingly, the finishing section has a gasoutlet in fluid communication with a gas inlet of a finishing treatmentsection adapted to subject ammonia-containing off-gas of the finishingsection to treatment with an acidic scrubbing liquid.

This finishing section may for example be a prilling tower, granulationsection, pelletizing section, or a section or equipment based on anyother finishing technique. A granulation section may for example be afluidized bed-granulation, or a drum granulation, or a pan- granulation,or any other similar granulation device. The main function of thisfinishing section is the conversion of a urea liquid, for example ureamelt, into a stream of solidified particles. To transfer the urea fromthe liquid phase into the solid phase, the heat of crystallization hasto be removed. Moreover, usually some sensible heat is removed from thesolidified urea particles, in order to cool them to a temperature thatis suitable for further processing and handling, including safe andcomfortable storage and transport of this final product. The resultingtotal removal of heat in the finishing section is usually done byevaporation of water and/or by cooling with air. For water evaporation,the water enters the finishing section either as part of the urealiquid, or is sprayed as liquid water at an appropriate place in thefinishing process. Usually most of the heat is removed by cooling withair. Usually an amount of air equal to 3-30 kg of air per kg of finalsolidified product is applied, preferably 3-10 kg. This is the typicaloff-gas of the finishing section. In the finishing section the air comesinto direct contact with the urea melt and with the solidified ureaparticles. This leads to contamination of the air with urea dust andammonia. Depending on the nature of the finishing section(grilling/granulation, type of granulation, conditions selected ingranulation), the amount of urea dust present in the air may varywidely, values in the range of 0.05% to 10% by weight (with respect tothe final product flow) having been observed. For a finishing sectionbased on granulation, the amount of dust more typically is in a range offrom 2% to 8% by weight. Urea in the finishing off-gas is mainly presentas urea dust comprising particles with a diameter less than 500 μm, witha large fraction of particles smaller than 10 μm, such as sub-micronparticles. Generally, this dust is carried along by the air stream whenthe air stream leaves the finishing section as off-gas. This presence ofurea dust in the finishing off-gas usually makes a treatment comprisingurea dust removal desirable, either for environmental or for efficiencyconsiderations, before the air can be vented into the atmosphere. Theremoval of urea dust is challenging per se, since the amounts of off-gas(mainly air) are enormous, whilst the concentration of urea dust is low.An example airstream is of the order of 750 000 Nm³/h. A typicalconcentration of urea dust therein is about 2 wt. %. Further, part ofthe urea dust is of a submicron size. Satisfying current standardsimplies the need to remove a major part of this submicron dust.

The solidification process may also comprise pelletizing, for example asdescribed in WO 2006/111331. In such process, urea-comprising particlesare produced in a pelletizer, comprising a feeding device, a belt and adevice to remove the formed pellets from the belt, by feeding a ureacontaining liquid stream to the feeding device from which droplets ofthe liquid are dosed to the belt, whereon the urea-containing dropletssolidify and cool to a temperature of <55° C. The formed urea-containingparticles are removed from the belt.

In the process of the invention, a finishing treatment section isinvolved having a gas inlet in fluid communication with a gas outlet ofthe finishing section, adapted to subject ammonia-containing off-gas ofthe finishing section (finishing off-gas) to treatment with an acidicscrubbing liquid. The treatment with an acidic scrubbing liquid resultsin removal of at least part of the urea dust and/or ammonia. Hence, theprocess may comprise scrubbing finishing off-gas. Scrubbing for examplecomprises adding scrubbing liquid into a gas stream, usually incounter-current flow.

The finishing treatment section comprises at least one scrubber forscrubbing with acidic scrubbing liquid. The finishing treatment sectionmay comprise additional scrubbers for scrubbing with acidic scrubbingliquids or other liquids.

Suitable types of scrubbers include for example venturi scrubbers,packed bed scrubbers, impingement scrubbers, and sieve tray scrubbers.In a venturi scrubber the effluent gas is forced or drawn through aventuri tube having a narrow “throat” portion. As the air moves throughthe throat it is accelerated to a high velocity. A scrubbing liquid inthe form of droplets, typically of water, is added to the venturi,usually at the throat, and enters the gas flow. The water droplets usedare generally many orders of magnitude larger than the contaminantparticles to be collected and, as a consequence, accelerate at adifferent rate through the venturi. The differential acceleration causesinteractions between the water droplets and the contaminant particles,such that the contaminant particles are collected by the water droplets.Aerosol droplets of AN off-gas can be considered as contaminantparticles for the purpose of their removal in a venturi scrubber. Thecollection mechanisms involve, primarily, collisions between theparticles and the droplets and diffusion of particles to the surface ofthe droplets. In either case, the particles are captured by thedroplets. Depending on the size of the contaminant particles, one or theother of these mechanisms may predominate, with diffusion being thepredominant collection mechanism for very small particles, and collisionor interception being the predominant mechanism for larger particles. Aventuri scrubber can also be efficient at collecting highly solublegaseous compounds by diffusion. A detailed description of thesescrubbing mechanisms is discussed in Chapter 9 of Air Pollution ControlTheory, M. Crawford, (McGraw-Hill 1976).

The finishing treatment section may comprise a single venturi scrubberor a plurality of venturi scrubbers. Further, one or more venturiscrubbers can themselves comprises one or more venturi tubes.

A venturi scrubber usually comprises three sections: a convergingsection, a throat section, and a diverging section. The inlet gas streamenters the converging section and, as the area decreases, gas velocityincreases. Liquid is introduced either at the throat or at the entranceto the converging section.

The inlet gas, forced to move at extremely high velocities in the smallthroat section, shears the liquid from its walls, producing an enormousnumber of very tiny droplets. Particle and gas removal occur in thethroat section as the inlet gas stream mixes with a fog of tiny liquiddroplets. The inlet stream then exits through the diverging section,where it is forced to slow down.

In case of a finishing treatment section comprising a scrubber, thetreatment section may comprise sections for one or more pre-treatmentsor post-treatments. For instance, a venturi scrubbing method asdescribed in WO 2015/002535 can be used. Such method comprises quenchingoff-gas to a temperature below about 45° C. and/or to a temperaturedecrease of at least 50° C., and subjecting the quenched off-gas toscrubbing using at least one venturi scrubber. Quenching comprisesadding aqueous quenching liquid to a gas stream, preferably by spraying,more preferably using an atomization nozzle, such as through a jetnozzle, for example co-currently with the gas stream. Quenchinggenerally provides a liquid saturation near equilibrium. Preferably thequenching stream has a temperature of below 45° C., more preferablybelow 40° C., most preferably below 35° C. The typical air temperatureof the off-gas exiting a finishing section of a urea plant, such as influid bed granulation, is about 110° C. After quenching, the temperatureis preferably below 45° C. Accordingly, the temperature of the gasstream is lowered by typically more than 50° C., preferably more than60° C., and most preferably more than 65° C. Preferably, the liquid issprayed in such a way and consistency that liquid droplets are formedthat are so small that the droplets evaporate quickly and a liquidsaturation in the vapor near equilibrium is reached within a short time,for example with a droplet size of less than 700 μm or less than 500 μm,or less than 100 μm.

In addition, a method such as described in WO 2015/072854 can be used.In such method, the gas stream is subjected to quenching in at least twostages in series, using an upstream quenching liquid and a downstreamquenching liquid, with the terms upstream and downstream being definedwith reference to the flowing direction of the gas stream, whereinsoluble particulate matter dissolves in the aqueous quenching liquid andwherein the downstream quenching liquid has a lower concentration ofdissolved said particulate matter than the upstream quenching liquid. WO2015/002535 and WO 2015/072854 are herewith incorporated by reference.Such quenching may provide for condensation of water on particles to beremoved, thereby increasing their particle size, such that they arebetter removed in a venturi scrubber.

In a preferred embodiment, the finishing treatment section comprises aplurality of venturi scrubbers, operated in parallel. Preferably, thefinishing treatment section is so designed that these parallel venturitubes can be operated independently of each other, i.e. the number ofventuri tubes used at the same time, can be adapted during the processas desired.

A preferred venturi scrubber comprises a so-called MMV-section(micro-mist venturi). The MMV-section consists of multiple parallelventuris. In the MMV-section large quantities of liquid are sprayed inthe throat of the venturis co-current with the gas-flow through singlephase nozzles, creating a consistent and adjustable liquid droplet-size,typically in a range of from 50 μm to 700 μm. The liquid droplet size isone of the parameters that can be used to control the efficiency ofdust-removal.

A preferred system is that provided by Envirocare, comprising aquenching section and downstream thereof a MMV-section.

In a packed bed scrubber, separation is usually achieved by contactbetween the gas and the scrubbing liquid over a random packed bed. In animpingement scrubber, separation is usually achieved by inertia througha central impingement plate. For example, an off-gas stream enters theunit from the bottom and flows upward through a series of trays, eachcontaining perforations. Scrubbing liquid is introduced from above thetop tray and cascades downward to the lower trays. The gas stream passesthrough the perforations and accelerates. This results in a fluidizedzone of liquid and gas. An impingement scrubbing unit is usuallyfurnished with a final demister section. Yet a further option for thescrubber is a sieve tray scrubber. Herein, liquid gas contact occurs onthe sieve tray. For example, a liquid stream flows horizontally whilethe gas passes through the sieves.

Yet a further option is a finishing treatment section comprising a wetelectrostatic precipitator (WESP). Wet electrostatic precipitators ofthis general type are known and described in prior art patents includingU.S. Pat. Nos. 1,339,480; 2,722,283; 4,389,225; 4,194,888; 6,106,592;and the prior art discussed and cited therein.

The finishing treatment section preferably comprises two parts inseries, a part for scrubbing with acidic scrubbing liquid and a ureadust removal part. The parts are optionally separate compartments. Thedust removal part, such as a venturi scrubber, is preferably upstream(with respect to the off-gas stream) of the part for scrubbing withacidic scrubbing liquid.

In one embodiment, in addition to the AN plant, also a calcium ammoniumnitrate plant is present. Calcium ammonium nitrate or CAN, also known asnitro-limestone, is a widely used inorganic fertilizer. One variety ofcalcium ammonium nitrate is made by adding powdered limestone/calciumcarbonate to ammonium nitrate; another, fully water-soluble version, isa mixture of calcium nitrate and ammonium nitrate, which crystallizes asa hydrated double salt: 5Ca(NO3)2.NH4NO3.10H2O.

The finishing section of the CAN plant (in either process) produces aCAN off-gas, gas which can also be scrubbed in the finishing treatmentsection. The finishing treatment section wherein also the CAN off-gas isscrubbed, comprises 2 separate compartments in series for scrubbing withacidic scrubbing liquid, designed to allow the spent scrubbing liquidsof each compartment to be used differently. Generally, the compartmentfor scrubbing CAN off-gas is positioned downstream of the compartmentfor scrubbing AN off-gas.

The scrubber may comprise an overflow of the acidic scrubber part. Theacidic scrubbing is for removing ammonia.

As mentioned above, the utilized scrubbing liquid from the finishingtreatment section, can be sent to the mixing unit for the UAN synthesis.In the event of the presence of a CAN plant, this will generally requirefirst removing a calcium containing salt solution. In an interestingembodiment, said utilized scrubbing liquid can itself be used as a UANproduct stream. Accordingly, the scrubbing liquid can be merged with aUAN product stream, or it can be stored in a tank for later use.

In a preferred embodiment, ammonia-containing off-gas resulting from theproduction of ammonium nitrate is combined with ammonia-containingoff-gas of the finishing section to provide a combined gas stream andthe combined gas stream is subjected to scrubbing to remove droplets andparticles and subsequently to scrubbing with acidic scrubbing liquid insaid finishing treatment section.

If the finishing treatment section comprises one or more venturiscrubbers, the gaseous stream of AN off-gas is preferably introducedinto the finishing treatment section upstream of one or more, or all, ofthe venturi scrubbers, in particular upstream of the venturi tubes. Thiscan help to scrub entrainments from the gas flow. Optionally, the ANoff-gas is supplied into or upstream of a quenching step upstream of aventuri step. This can advantageously provide for growth of theparticles entrained in the AN off-gas.

The finishing treatment section for example comprises a venturi scrubberor a WESP, more preferably a combination of, in series, a wet scrubber(such as a tray scrubber) and, a venturi scrubber, with the venturiscrubber more preferably downstream of the wet scrubber. Morepreferably, the venturi scrubber comprises a plurality of venturi tubesin parallel. In another preferred embodiment, a WESP is positioneddownstream of the wet scrubber, or downstream of the Venturi scrubber,or most preferably in series after the wet scrubber and the Venturiscrubber. Optionally, the finishing treatment section comprises ascrubber for scrubbing with acidic scrubbing liquid downstream of theventuri scrubber and/or WESP.

The invention also provides as mentioned a method of modifying a plant,wherein the method comprises adding a connection for fluid communicationbetween an outlet for off-gas from said ammonium nitrate section and agas inlet of said finishing treatment section, such as piping or tubing.

The method is usually a method for modifying or revamping a pre-existingplant. The plant can be of various types and the method may compriseadditional steps prior to adding said connection. The method may be amethod for modifying a plant for producing urea ammonium nitrate and asolid urea product. Preferably, the plant comprises, before themodification, a condensation unit for condensing a part of the off-gasfrom said ammonium nitrate section and the method involves eliminatingor bypassing said condensation unit.

The method may also be part of a method for modifying an ammoniumnitrate plant, wherein the method further comprises adding a ureaproduction unit and adding a UAN production section. Preferably, theplant comprises, before the modification, a condensation unit forcondensing a part of the off-gas from said ammonium nitrate section andthe method involves eliminating or bypassing said condensation unit. Themethod may also be part of a method for modifying a plant for theproduction of UAN comprising a urea production unit, wherein the methodfurther comprises expansion of the urea production unit by adding afinishing section.

The method may also be part of a method for modifying a urea plantincluding a finishing section, wherein the method further comprisesadding an ammonium nitrate section.

The invention also relates to a system for the production of at leasturea and urea ammonium nitrate, comprising as mentioned (a) a ureaproduction unit (b) an ammonium nitrate section, (c) a urea ammoniumnitrate section and (d) a finishing treatment section wherein saidfinishing treatment section comprises a gas inlet in fluid communicationwith said outlet for off-gas of said ammonium nitrate section.

Preferably, the inlet for receiving urea liquid of said urea ammoniumnitrate section is in fluid connection with said urea production unitcomprising a finishing section. Preferably, the urea production unitcomprises a high pressure urea synthesis section comprising a highpressure stripper and a carbamate condenser and a urea reactor, or anintegrated carbamate condenser and a urea reactor, wherein said ureaproduction unit further comprises a recovery section and an evaporationsection. Preferably, the ammonium nitrate section comprises an inlet influid communication for off-gas with one or more of said high pressuresynthesis section, recovery section and evaporation section.

Preferably, the finishing treatment section comprises an outlet for aliquid stream in fluid communication with a point downstream of saidoutlet for ammonium nitrate solution, such as with an inlet of saidmixing unit. Preferably, such system is also suitable for the productionof a DEF product, wherein the urea production unit comprises a dividerfor dividing a stream of urea liquid in at least two streams, and a unitfor adding an additive to one of said streams downstream of said dividerand upstream of a finishing section, and a unit for preparing a DEFproduct from the other stream.

Preferably, the treatment section comprises a venturi scrubber, morepreferably an MMV scrubber.

FIG. 1 shows a process scheme for a non-limiting example of a processand system according to the invention. Nitric acid 1 and ammonia gas 2are reacted to provide aqueous ammonium nitrate solution 3 in anammonium nitrate section A, thereby also yielding AN off-gas 4. Aqueousammonium nitrate solution 3 is supplied to urea ammonium nitrate sectionB and off-gas 4 from ammonium nitrate section A is supplied to afinishing treatment section C. In urea production unit D urea isproduced from carbon dioxide 5 and ammonia 6, yielding a urea liquid 9(purified aqueous urea solution and/or concentrated urea liquid) whichis supplied to finishing section E. Optionally, a part of the urealiquid 7 is supplied to urea ammonium nitrate section B. It is alsopossible that section B receives urea liquid from another ureaproduction unit, instead or in combination with urea liquid 7. Insection B, urea liquid is mixed with aqueous ammonium nitrate solution 3to provide UAN stream 8. The system also comprises finishing section Ewherein concentrated urea liquid 9 from urea production unit D issolidified Finishing section E yields a solid urea product 10 and is forexample a granulator or prilling tower. Finishing section E usesdrying/cooling air 11 and provides ammonia-containing off-gas 12 that issupplied to finishing treatment section C adapted to subject off-gas 12to treatment with an acidic scrubbing liquid 13 to give a cleanedoff-gas 14. Optionally, spent scrubbing liquid 15 is recycled bysupplying it to urea ammonium nitrate section B. Optionally, off-gasfrom urea production unit D is supplied to finishing treatment section Cas a stream 16 and/or to ammonium nitrate section A as stream 17.

The term “fluid communication” includes any connection any connectionbetween a first part or section of a plant and a second part or sectionof a plant via which fluids, notably liquids, can flow from the firstpart of the plant to the second part of the plant. Such fluidcommunication is typically provided by piping systems, hoses, ducts,pumps, or other devices well-known to the skilled person for thetransportation of fluids. The fluid communication can be direct fluidcommunication, such as any of the foregoing without involving anyfurther equipment other than the fluid transportation devicesthemselves. The fluid communication can also be indirect, wherein thefluid may be transported via piping system, hoses, ducts or pumps, andalso including other equipment such as strippers or reactors. As usedherein, the terms “inlet” and “outlet” can also be used for intermediatestreams.

The term “liquid stream” includes suspensions and dispersions andgenerally relates to a fluid stream comprising a continuous liquidphase. The term “gaseous stream” does not exclude the presence ofentrained droplets and particles.

1. A method for the production of urea ammonium nitrate comprising (a)subjecting ammonia and carbon dioxide to urea forming conditions so asto obtain an aqueous urea solution, (b) purifying the aqueous ureasolution in a recovery section to remove residual ammonium carbamate soas to form a purified aqueous urea solution, and optionally subjectingat least part of the purified aqueous urea solution to evaporation so asto form concentrated urea liquid, (c) subjecting ammonia and nitric acidto ammonium nitrate forming conditions so as to form an aqueous ammoniumnitrate solution; (d) combining said aqueous ammonium nitrate solutionand at least a part of the purified aqueous urea solution and/orconcentrated urea liquid in a urea ammonium nitrate section so as toobtain an aqueous solution of urea ammonium nitrate; (e) treatingammonia-containing off-gas resulting from the production of ammoniumnitrate with acidic scrubbing liquid in a finishing treatment sectionhaving a gas inlet in fluid communication with a gas outlet of afinishing section of a urea production unit, wherein the finishingsection is adapted to solidify urea liquid, and wherein said finishingtreatment section is adapted to subject ammonia-containing off-gas ofthe finishing section to treatment with an acidic scrubbing liquid. 2.The method of claim 1, comprising recycling all or part of the scrubbingliquid utilized in the finishing treatment section to the urea ammoniumnitrate section.
 3. The method of claim 1, comprising sending all orpart of the scrubbing liquid utilized in the finishing treatment sectiondirectly to the aqueous solution of urea ammonium nitrate.
 4. The methodof claim 2, wherein in addition a Diesel Exhaust Fluid product isprepared, the process comprising dividing a stream of urea liquid of theurea production unit having the finishing section in at least twostreams, adding an additive to one of said streams downstream of saiddividing, and preparing a Diesel Exhaust Fluid product from the otherstream, and wherein the stream with said additive is subjected tosolidification in the finishing section.
 5. The method of claim 1,wherein also the ammonia containing off-gas from the urea productionunit and/or the urea ammonium nitrate section is sent to a gas inlet ofthe finishing treatment section.
 6. The method of claim 1, whereinammonia-containing off-gas resulting from the production of ammoniumnitrate is combined with ammonia-containing off-gas of the finishingsection to provide a combined gas stream and the combined gas stream issubjected to scrubbing to remove droplets and particles and subsequentlyto scrubbing with acidic scrubbing liquid in said finishing treatmentsection.
 7. The method of claim 1, wherein the process does not involvecondensing off-gas resulting from the production of ammonium nitratebetween the urea ammonium nitrate section and the finishing treatmentsection.
 8. The method of claim 1, the process further comprisingproducing Calcium Ammonium Nitrate granules by (f) subjecting a part ofthe aqueous ammonium nitrate solution of step (c) and a calcium saltstream, such as calcium carbonate or calcium nitrate, to a CalciumAmmonium Nitrate finishing unit so as to form Calcium Ammonium Nitrategranules and a Calcium Ammonium Nitrate off-gas, said off-gas containingammonia, ammonium nitrate and entrained Ca-containing particles; (g)feeding said off-gas to the finishing treatment section wherein saidfinishing treatment section is adapted to treat said off-gas with anacidic scrubbing liquid thereby forming a Ca-containing salt solutionbefore recycling the scrubbing liquid to the UAN section.
 9. Anapparatus for the production of at least urea and urea ammonium nitrate,preferably suitable for the method of claim 1, comprising: (a) a ureaproduction unit comprising a finishing section, wherein the finishingsection has a gas outlet for ammonia-containing off-gas, (b) an ammoniumnitrate section for producing ammonium nitrate in fluid communicationwith a source of nitric acid and a source of ammonia, having an outletfor aqueous ammonium nitrate solution and an outlet for off-gas, (c) aurea ammonium nitrate section comprising a unit having an inlet in fluidconnection with said outlet for aqueous ammonium nitrate solution and aninlet for receiving urea liquid, for combining said ammonium nitratesolution and said urea liquid, and having an outlet for urea ammoniumnitrate solution, and (d) a finishing treatment section having a gasinlet in fluid connection with said gas outlet for off-gas of saidfinishing section, adapted to subject ammonia-containing off-gas of thefinishing section to treatment with an acidic scrubbing liquid, whereinsaid finishing treatment section comprises a gas inlet in fluidcommunication with said outlet for off-gas of said ammonium nitratesection.
 10. The apparatus of claim 9, wherein said inlet for receivingurea liquid of said urea ammonium nitrate section is in fluid connectionwith said urea production unit comprising a finishing section, whereinsaid urea production unit comprises a high pressure urea synthesissection comprising a high pressure stripper and a carbamate condenserand a urea reactor, or an integrated carbamate condenser and a ureareactor, wherein said urea production unit further comprises a recoverysection and an evaporation section, and wherein said ammonium nitratesection comprises an inlet in fluid communication for off-gas with oneor more of said high pressure synthesis section, recovery section andevaporation section.
 11. The apparatus of claim 9, wherein saidfinishing treatment section comprises an outlet for a liquid stream influid communication with a point downstream of said outlet for aqueousammonium nitrate solution, such as with an inlet of said unit.
 12. Theapparatus of claim 11, which is also suitable for the production of aDEF product, wherein the urea production unit comprises a divider fordividing a stream of urea liquid in at least two streams, and a unit foradding an additive to one of said streams downstream of said divider andupstream of a finishing section, and a unit for preparing a dieselexhaust fluid product from the other stream.
 13. The apparatus of claim9, wherein said treatment section comprises a venturi scrubber.
 14. Theapparatus of claim 9, comprising a calcium ammonium nitrate productionsection, said calcium ammonium nitrate production section comprising aninlet for ammonium nitrate in fluid communication with an outlet forammonium nitrate from the ammonium nitrate section, wherein the calciumammonium nitrate production section comprises a granulation sectionhaving an outlet for off-gas in fluid communication with a gas inlet ofthe finishing treatment section.
 15. A method of modifying a plant,wherein the plant comprises an ammonium nitrate section for reactingammonia and nitric acid under ammonium nitrate forming conditions, afinishing section adapted to solidify a urea liquid, and a finishingtreatment section having a gas inlet in fluid communication with a gasoutlet of said finishing section, adapted to subject ammonia-containingoff-gas of the finishing section to treatment with an acidic scrubbingliquid, wherein the method comprises adding a connection for fluidcommunication between an outlet for off-gas from said ammonium nitratesection and a gas inlet of said finishing treatment section, such aspiping or tubing. 16-18. (canceled)